Require one of two values to allow a specific state

Hi,

Working on TFS PBI workflow.  I need to allow the user to select a state only when another field has one of two values.  Here's the scenario:

Our state field has a Pre-Production value.  When the user selects this state, I only want to allow the form to be saved if the Testing Status field is either "No Testing Required" or Tested.  The Testing Status field is a drop down.  

I'm somewhat of a newbie to XML so please bear with me.  It appears that the WHEN condition might work but I'm not sure how to set it up.  Should it be on the Transition element or the Field element?  And how does one set it up this condition?


TIA for your help.